In a hydrogen production system that produces hydrogen with a high purity of 99.9% or more, synthesis gas is produced by partial combustion of a raw material such as coal in a high-temperature gas furnace, and hydrogen is separated from the synthesis gas. The synthesis gas contains, in addition to hydrogen (H2) as a main component, carbon monoxide (CO), carbon dioxide (CO2), and water (H2O), hydrogen sulfide (H2S). In a process intended to merely produce hydrogen, hydrogen sulfide is separated together with carbon dioxide that is an acidic gas by a chemical absorption method (e.g., an absorption method using an amine) or a physical absorption method (e.g., an absorption method using a polymer solution).
Recently, there has been a growing need for CCS (Carbon dioxide Capture and Storage) or EOR (Enhanced Oil Recovery), but it has been pointed out that there is a problem that hydrogen sulfide contained in separated carbon dioxide corrodes a piping. In order to avoid this problem, separation of hydrogen sulfide is required to reduce the concentration of hydrogen sulfide in carbon dioxide to a low level of, for example, 10 ppm or less.
In order to separate hydrogen sulfide contained in carbon dioxide, a technique has been developed in which hydrogen sulfide is selectively separated using an aqueous triethanolamine solution (Patent Document 1). However, in this technique, most of supplied carbon dioxide is absorbed by the aqueous triethanolamine solution together with hydrogen sulfide. Therefore, this technique is unsatisfactory in that the amount of carbon dioxide recovered is reduced and the amount of heat for separating hydrogen sulfide is increased.
Further, another technique has also been developed in which hydrogen sulfide is selectively separated using an aqueous solution of a sterically-hindered amine (Patent Document 2). This technique is an invention intended to improve a reduction in the amount of hydrogen sulfide absorbed due to a large difference in concentration between carbon dioxide and hydrogen sulfide. Patent Document 2 states that hydrogen sulfide can be selectively absorbed by this technique. However, also in this technique, the fact remains that carbon dioxide is absorbed together with hydrogen sulfide at the same time. Therefore, this technique is unsatisfactory in that hydrogen sulfide is mixed into recovered carbon dioxide, the amount of heat for separating hydrogen sulfide is increased, and the amount of carbon dioxide recovered is reduced.
Further, an attempt has been made to remove hydrogen sulfide by bringing biogas containing carbon dioxide and hydrogen sulfide into contact with zeolite having moisture previously adsorbed thereto as a catalytic reaction suppressing agent (Patent Document 3). However, in this technique, the amount of water contained in zeolite needs to be maintained at 0.2 to 3.3 wt % and desorption of hydrogen sulfide needs to be performed under conditions of a high temperature of 200° C. or more. Therefore, this technique still has problems such as difficulty in control of moisture and large consumption of regenerated energy.